New factions aren't slated for this year, right now it's all about adding in the Termies and Elites they promised us
Hmm... As much as I'd like to strut around as the most cheerful Guard regiment in 40k, the Death Korps of Krieg, I don't see how you'd easily implement the Guard as a "story" faction when they fight Marines, it didn't work in Dawn of War and it wont work here. I'd feel pity for the Tau if they were introduced into this melee Meta... I could easily see the Dark Eldar causing untold amounts of Rage and then an eternal nerf considering many if not all of their units have DOT weapons. Necrons would be interesting, high damage/pen weapons and when killed I would imagine instead of bleeding out, your bar would fill up and when complete, you stand up and start fighting again, you could also have a Lord throw in some orb like the Apoths grenade to speed up the revive. This would force opponents to finish off/execute the Necron. You would offset this with a tragically slow movement speed and less utility with weapons etc.
I have to say it is a little sad that Tyranids got used up in PvE. Could you imagine how cool it would be to customize and mod out all your arms for weapons etc? CARNIFEX PLAYER CONTROLLED TANKS? YES PLEASE!
In theory the devs said we can expect PvE with orkish enemies in the future so being also npc shouldn't exclude player controlled ones; nor has any real reason to.
I'd like to say 'Imperial Guard because there are like a million Astartes on this planet, where the hell is the hammer of the Emperor.' Or 'Dark Eldar, because there are four factions to steal shit from on Arkhona'. But let's keep it super real, Arkhona is obviously a tomb world.
Tau would get obliterated. Their infantry weapons are no better than bolters, and given the size of the maps, they would be mauled to death by melee. Necrons I could see. Ultimately though, could we first flesh out and get right the factions that we have already? ¬_¬
Lore isn't stopping the Tau from showing up, as Battlefleet Gothic has shown. GW let's developers make shit up if they pony up lol.
I would like to see Tau, I don't think the imperial guard would be any good. I can see them being a huge balancing issue. Tau have always interested me for some reason. Like a more advanced SM faction.
Necrons are the ones that make more sense because they may be asleep in the world and the artifact could be linked with the old ones. That would make them MAD enough to get in the fight. Also gameplay wise are pretty good. Make slow but durable people. They have a lot of power but lack of persecution mechanics.